feat(sdk): Optimise how LinkedChunk::insert_gap_at works when inserting at first position
#3251
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Hywan
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poljar
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… position. Imagine we have this linked chunk: ```rust assert_items_eq!(linked_chunk, ['a'] [-] ['b', 'c'] ['d', 'e', 'f']); ``` Before the patch, when we were running: ```rust let position_of_d = linked_chunk.item_position(|item| *item == 'd').unwrap(); linked_chunk.insert_gap_at((), position_of_d)?; ``` it was taking `['d', 'e', 'f']` to split it at index 0, so `[]` + `['d', 'e', 'f']`, and then was inserting a gap in between, thus resulting into: ```rust assert_items_eq!(linked_chunk, ['a'] [-] ['b', 'c'] [] [-] ['d', 'e', 'f']); ``` The problem is that it creates a useless empty chunk. It's a waste of space, and rapidly, of computation. When used with `EventCache`, this problem occurs every time a backpagination occurs (because it executes a `replace_gap_at` to insert the new item, and then executes a `insert_gap_at` if the backpagination contains another `prev_batch` token). With this patch, the result of the operation is now: ```rust assert_items_eq!(linked_chunk, ['a'] [-] ['b', 'c'] [-] ['d', 'e', 'f']); ``` The `assert_items_eq!` macro has been updated to be able to assert empty chunks. The `test_insert_gap_at` has been updated to test all edge cases.
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| // insert a new gap chunk, otherwise it would create an empty current items | ||
| // chunk. Let's handle this case in particular. | ||
| // | ||
| // Of course this optimisation applies if there is a previous chunk. Remember |
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From where should we remember this invariant? The docs don't really explain why a gap can't be the first chunk 😅.
I just saw an INVARIANT tag in the constructor which says the same, though doesn't explain the why.
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Because it's easier ^^. I'll write a longer documentation if LinkedChunk proved to be OK, but for now, we are experimenting with it. I'm postponing these details for next week.
Codecov ReportAttention: Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## main #3251 +/- ##
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+ Coverage 83.66% 83.67% +0.01%
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Files 236 236
Lines 24470 24480 +10
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+ Hits 20472 20483 +11
+ Misses 3998 3997 -1 ☔ View full report in Codecov by Sentry. |
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Imagine we have this linked chunk:
Before the patch, when we were running:
it was taking
['d', 'e', 'f']to split it at index 0, so[]+['d', 'e', 'f'], and then was inserting a gap in between, thus resulting into:The problem is that it creates a useless empty chunk. It's a waste of space, and rapidly, of computation. When used with
EventCache, this problem occurs every time a backpagination occurs (because it executes areplace_gap_atto insert the new item, and then executes ainsert_gap_atif the backpagination contains anotherprev_batchtoken).With this patch, the result of the operation is now:
The
assert_items_eq!macro has been updated to be able to assert empty chunks. Thetest_insert_gap_athas been updated to test all edge cases.EventCachefully usesRoomEvents/LinkedChunk#3230.